Oxygen-starved oceans delayed recovery after 'Great Dying'

Chronically low levels of oxygen throughout the oceans hampered the recovery of life after the 'Great Dying', the most catastrophic die-off in Earth's history, a new study has found.
Global ecosystems collapsed as some 90 per cent of species perished in this Permian-Triassic extinction event 250 million years ago.
For the first time, researchers from Stanford University in US have shown that ocean anoxia, or oxygen deficiency, was a global rather than an isolated phenomenon.
The study paints a dire portrait of how anoxic conditions reduced seawater oxygen levels by 100-fold at the onset of the mass extinction.
Oxygen levels then slowly rose, only returning to pre-extinction levels after 5 million years, corresponding to when the climate became more stable and life regained its former diversity.

A devastating confluence of geological events is thought to have triggered the Great Dying a quarter billion years ago, including a massive eruption of climate-changing carbon dioxide from volcanoes associated with the Siberian Traps.

Numerous studies have pointed to ocean anoxia playing a role both in the actual extinction event as well as its prolonged recovery phase.
However, these studies could not reliably testify beyond local conditions to the world's waters as a whole.
Key to the new study was identifying an anoxic signal that could be traced independently of regional circumstances.
For that, researchers used a new technique using uranium, preserved in limestone, that had once been dissolved and mixed evenly throughout the oceans.

"Because uranium is slowly cycled through the ocean, these records are thought to represent global changes in oxygenation," said Kimberly Lau from Stanford University.
The dissolved uranium became trapped in seabed rocks when microbes chemically modified it into an insoluble form. Some microbes also utilise iron and sulphur to generate energy, creating minerals that further pull uranium out of the water.
Uranium atoms naturally occur in two isotopes, or versions with differing numbers of neutrons, and these isotopes behave differently in chemical reactions.
Conveniently for the sake of gauging anoxia, the rates of these various reactions involving uranium change are based on available oxygen.

"Our results suggest a unified explanation for biological and biogeochemical observations stemming from the most severe biotic crisis in Earth's history," said Lau.
With this technique, researchers obtained rock samples from two widely separated sites, now located in modern-day China and Turkey.
The samples of ancient marine sediments covered a wide time interval of the Permian-Triassic boundary, and given geographical separations of thousands of miles, attest to global, rather than local seawater characteristics.
The findings were published in the journal Proceedings of the National Academy of Sciences.